1. Field of the Invention
The present invention relates to a video signal processing apparatus which is adapted for processing an input video signal in a desired mode and producing a printable output video signal.
2. Description of the Prior Art
Technical development relative to a still print image processing system is currently in progress, wherein a video signal of one frame representing a still image and obtained from a video source such as a television camera, video disk, still camera or the like is input to an image processor, which then processes the input video signal in various stages to generate a desired print image signal, and such processed image signal is supplied to a printing means so as to be printed.
Since such a still image is continuously displayed for a long time, some image quality deterioration inclusive of luminance and chrominance nonuniformity is prone to be conspicuous as compared with a moving image, and therefore it is necessary to eliminate the video signal noise which causes such image quality deterioration. In the conventional techniques contrived for elimination of the image noise, there are known an intra-window averaging and a smoothing with a low-pass filter or the like in a spatial frequency region. As the techniques for eliminating the video signal noise without impairing any important information of the image relative to its edges and so forth, there are further known a selective local averaging and a special process for removal of minute-amplitude components.
The selective local averaging is a technique of first detecting edges or segments and then averaging merely the dots where the detected edges or segments are not existent. Exemplary general-purpose methods proposed heretofore include a smoothing with retention of edges (e.g., CGIP, Vol. 9, No. 4, pp. 394-407, April 1979) repeated for elimination of noise, and a smoothing with local pixel selection. In addition thereto, a variety of algorithms are proposed with regard to exclusive operators for special subjects to be processed.
Special processing for elimination of minute-amplitude components is a technique of absorbing small signal fluctuations while retaining great-amplitude information relative to edges and segments. For example, there are known hysteresis smoothing which absorbs noise by hysteresis characteristics (e.g., CGIP, Vol. 8, No. 8, pp. 121-126, August 1978); a median filter capable of outputting a median or intermediate value in a window; and an E filter which performs low-pass filtering in a coordinate system dependent on the input signal amplitude and thereby eliminates only the low-amplitude components.
However, one of such conventional noise elimination techniques removes the minute signal fluctuations in local regions, but is incapable of removing the video signal noise which causes color nonuniformity and so forth in a relatively wide area such as a planar portion of a building or a background in a still picture where color variations are minimal.